From: Dan Carpenter <dan.carpenter@oracle.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Markus Elfring <Markus.Elfring@web.de>,
kernel-janitors@vger.kernel.org,
Alexander Shishkin <alexander.shishkin@linux.intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
Joe Perches <joe@perches.com>, Kees Cook <keescook@chromium.org>,
Steven Rostedt <rostedt@goodmis.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] string.h: Mark 34 functions with __must_check
Date: Wed, 9 Oct 2019 21:45:18 +0300 [thread overview]
Message-ID: <20191009184518.GF13286@kadam> (raw)
In-Reply-To: <CAKwvOd=Jkd_qJULB+i1u31VJAex6KB=wFAyXO04V0UcAAEZeXw@mail.gmail.com>
On Wed, Oct 09, 2019 at 09:31:41AM -0700, Nick Desaulniers wrote:
> On Wed, Oct 9, 2019 at 7:30 AM Dan Carpenter <dan.carpenter@oracle.com> wrote:
> >
> > On Wed, Oct 09, 2019 at 04:21:20PM +0200, Rasmus Villemoes wrote:
> > > On 09/10/2019 15.56, Dan Carpenter wrote:
> > > > That's because glibc strlen is annotated with __attribute_pure__ which
> > > > means it has no side effects.
> > >
> > > I know, except it has nothing to do with glibc headers. Just try the
> > > same thing in the kernel. gcc itself knows this about __builtin_strlen()
> > > etc. If anything, we could annotate some of our non-standard functions
> > > (say, memchr_inv) with __pure - then we'd both get the Wunused-value in
> > > the nonsense cases, and allow gcc to optimize or reorder the calls.
> >
> > Huh. You're right. GCC already knows. So this patch is pointless like
> > you say.
>
> Is it? None of the functions in include/linux/string.h are currently
> marked __pure today.
I've already embarrassed myself with my ignorance once so I may as well
keep talking now... GCC did complain about the unused result even
though we don't declare them as __pure. So GCC rule must have this rule
built in.
We were discussing in a different thread that standard says that
memcpy() pointers can't be NULL (even when we're copying zero bytes) so
GCC will assume that's true. If you have:
memcpy(foo, bar, 0);
if (foo)
*foo = 0;
GCC will sometimes remove the condition. This doesn't affect the kernel
because we use -fno-delete-null-pointer-checks.
https://groups.google.com/forum/#!msg/syzkaller-netbsd-bugs/8B4CIKN0Xz8/wRvIUWxiAgAJ
Weird, huh?
regards,
dan carpenter
next prev parent reply other threads:[~2019-10-09 18:48 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-09 12:14 [PATCH] string.h: Mark 34 functions with __must_check Markus Elfring
2019-10-09 13:26 ` Rasmus Villemoes
2019-10-09 13:56 ` Dan Carpenter
2019-10-09 14:21 ` Rasmus Villemoes
2019-10-09 14:30 ` Dan Carpenter
2019-10-09 16:31 ` Nick Desaulniers
2019-10-09 18:45 ` Dan Carpenter [this message]
2019-10-10 7:20 ` Rasmus Villemoes
2019-10-09 16:37 ` Nick Desaulniers
2019-10-09 16:42 ` Markus Elfring
2019-10-11 5:15 ` Searching for missing variable checks Markus Elfring
2019-10-09 15:09 ` [PATCH] string.h: Mark 34 functions with __must_check Steven Rostedt
2019-10-09 16:13 ` Nick Desaulniers
2019-10-09 16:27 ` Steven Rostedt
2019-10-09 16:40 ` Nick Desaulniers
2019-10-09 17:04 ` Markus Elfring
2019-10-09 17:33 ` Nick Desaulniers
2019-10-09 18:06 ` Markus Elfring
2019-10-09 16:38 ` [PATCH] " Joe Perches
2019-10-09 17:33 ` Nick Desaulniers
2019-10-10 14:27 ` David Sterba
2019-10-10 14:34 ` Joe Perches
2019-10-11 5:00 ` Markus Elfring
2019-10-10 15:46 ` [PATCH] " David Laight
2019-10-09 20:06 ` Markus Elfring
2019-10-10 5:29 ` Andy Shevchenko
2019-10-10 7:25 ` Markus Elfring
2019-12-21 9:30 ` Markus Elfring
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